The invention relates to a method for a local high-doping and contacting of a semiconductor structure, which represents a solar cell or a precursor of a solar cell and comprises a semiconductor substrate of a base doping type.
In photo-voltaic solar cells, based on a silicon semiconductor substrate, a plurality of approaches have been undertaken to achieve high effectiveness in the conversion of incident electromagnetic radiation into electric energy, on the one hand, and cost-effective industrial production, on the other hand. In particular, in solar cells in which an emitter and thus also a pn-junction separating the generated pairs of charge carriers or in the area of the front of the solar cell embodied for incident light typically the electric contacting of the base occurs via a metallic contacting layer arranged at the rear, which is connected in an electrically conductive fashion to the semiconductor substrate. Here, it is essential for yielding high effectiveness that the rear is effectively passivated, i.e. a low surface recombination velocity is achieved for the minority charge carriers, particularly in the area of the rear surface of the semiconductor substrate, as well as a contacting with low contact resistance.
For example, solar cell structures are known in which the rear of the semiconductor substrate is essentially covered entirely with a silicon nitride or silicon dioxide layer such that low surface recombination velocities are yielded. Only at contact points the passivating layer is punctually open and an electrically conductive connection to a metallic contacting layer is given. An increase in effectiveness is achieved by a local high-doping in the area of the contact points, because due to the local high-doping a reduction of the rear recombination and additionally a reduction of the contact resistance are yielded between the metallic contacting layer and the semiconductor substrate. An example for such a solar cell structure is the PERL-structure (passivated emitter, rear locally defused), as described in J. Benick, B. Hoex, G. Dingemans, A. Richer, M. Hermle, and S. W. Glunz “High-efficiency n-type silicon solar cells with front side boron emitter”, in Proceedings of the 24th European Photovoltaic Solar Energy Conference (Hamburg, Germany), 2009. This structure, which allows a rear contacting to yield high effectiveness, requires for its production, in addition to the supplementary diffusion to yield the high-doping areas, some additional photo-lithographic steps, so that an industrial implementation of this solar cell—structure is not practicable or at least very cost intensive.
A solar cell structure with low recombination velocities at the rear could be realized by the HIT-structure, which compared to the PERL-structure allows an easier industrial implementation. The HIT-structure is described in EP 1 187 223 A2 and M. Taguchi, Y. Tsunomura, H. Inoue, S. Taira, T. Nakashima, T. Baba, H. Sakata, and E. Maruyama, “High-efficiency HIT solar cell on thin (<100 μm) silicon wafer,” in Proceedings of the 24th European Photovoltaic Solar Energy Conference (Hamburg, Germany), 2009. At the rear of a HIT-solar cell a hetero-junction is formed by a layer structure being applied to the rear of the silicon semiconductor substrate, comprising a thin intrinsic layer comprising amorphous silicon, a thin doped layer of amorphous silicon, and a transparent, conductive oxide. The layer of intrinsic amorphous silicon ensures a highly efficient passivation of the rear and thus low recombination velocities for the minority charge carriers. However, simultaneously this layer must be sufficiently thin to allow a charge carrier traveling via tunnel processes into the doped layer located thereabove comprising amorphous silicon. This leads to the requirement for the layer thickness and the quality of the amorphous silicon layers to be very high, particularly regarding the precision during the application process of the layers with regards to the layer thickness and also the material quality. High effectiveness can be achieved with this solar cell concept only if these high requirements are ensured.